1. Field of the Invention
The present invention relates to a control system for a hybrid electric vehicle equipped with an engine and a motor.
2. Description of the Related Art
A hybrid electric vehicle equipped with an engine and an electric motor and capable of transmitting the driving force of the engine and that of the electric motor to the driving wheels of the vehicle has conventionally been developed and in practical use.
As one example of such a hybrid electric vehicle, a hybrid electric vehicle is proposed, for example, in Unexamined Japanese Patent Publication No. 5-176405 (hereinafter, referred to as Document 1), in which the rotary shaft of the electric motor is coupled between the output shaft of the clutch that connects/disconnects the driving force transmitted from the engine to the transmission and the input shaft of the transmission.
At the startup of the hybrid electric vehicle as disclosed in Document 1, the clutch is disengaged, and the electric motor is operated as a motor by power supply from the battery to start the vehicle only by the driving force of the electric motor. During the running of the vehicle after startup, the clutch is engaged, so that the driving force of the engine is transmitted through the transmission to the driving wheels.
While the vehicle is driven by the driving force of the engine, the torque required for the running of the vehicle is properly distributed to the engine and the electric motor, and the electric motor is operated as a motor so as to supplement the driving force. At the deceleration of the vehicle, the electric motor is operated as a generator to create a regenerative braking force, and the braking energy is converted to electric power to charge the battery.
In the hybrid electric vehicle thus constructed, if the storage rate (hereinafter, referred to as SOC) of the battery is reduced too low, in order to prevent the overdischarge of the battery, the battery is forcibly charged by operating the electric motor as a generator to recover the SOC. Contrary to this, if the SOC is raised too high, in order to prevent the overcharge of the battery, the electric motor is operated as a motor, and the torque distribution to the electric motor is increased. By so doing, the battery is forcibly discharged, and the SOC is reduced to a proper value.
Occasionally, in the hybrid electric vehicle thus constructed, the driving and braking forces produced by the electric motor are not required depending upon the operating state of the vehicle, such as during idle operation of the engine or during constant-speed driving. In that case, the electric motor is controlled so that the motor torque becomes zero, that is, so that the electric motor does not create the driving and braking forces at all.
However, in the situation where a synchronous motor is used as an electric motor, for example, even if the electric motor is controlled so that the motor torque becomes zero, the control accuracy of the electric motor cannot be sufficiently secured due to the properties of the electric motor. Therefore, it is difficult to maintain the torque to precisely zero, and very small amount of current flows between the battery and the electric motor.
Accordingly, if the control for controlling the motor torque to become zero, such as the idle operation of the engine and constant-speed driving, is continued for a long period of time, very small amount of current continuously flows between the battery and the electric motor. As a result, the SOC of the battery is reduced or increased by degree. If the SOC of the battery is excessively reduced or increased, in order to prevent the overdischarge or overcharge of the battery, a forced charge or discharge is carried out as already stated.
Consequently, in spite that the vehicle is in the state where the motor torque is not basically required, the kinetic energy of the electric motor and the electric energy of the battery are uselessly consumed since the current flows between the electric motor and the battery. Furthermore, the forced charge and discharge hamper the proper torque distribution between the engine and the electric motor, and degrade the operating efficiency of the engine. Moreover, during the forced charge and discharge, engine power is converted to electric energy, and the electric energy is consumed more than required. This also deteriorates energy efficiency and then lowers fuel economy.